LED lighting systems are becoming more prevalent as replacements for existing lighting systems. LEDs are an example of solid state lighting and are superior to traditional lighting solutions such as incandescent and fluorescent lighting because they use far less energy, are far more durable, operate longer, can be combined in red-blue-green arrays that can be controlled to deliver virtually any color light, and contain no lead or mercury. As LEDs replace the typical fluorescent light fixtures found in many workplaces, the present invention recognizes that it is important to cost effectively dissipate the heat generated by the LEDs used in these systems while enabling relatively simple physical retrofitting or replacement of existing lighting hardware.
One common fluorescent lighting fixture is a luminaire fixture 100 shown illustratively in FIG. 1. Fixture 100 may suitably comprise a 2′ by 4′ metal box or compartment 102 having a plurality of fluorescent bulbs 104, 106 and 108. While a 2′ by 4′ fluorescent fixture is discussed here as exemplary, it will be recognized that many other sizes of fluorescent fixture and various incandescent fixtures are also common. Each fluorescent bulb, such as bulb 108, is inserted in an electrical socket, and located within a reflective subassembly 210 as seen in greater detail in FIG. 2. The compartment 102 also has a reflective back surface, such as a white painted interior surface and a plastic cover mounted in a hinged door (not shown) which swings open to allow the bulbs to be easily accessed and changed. Such a fixture with its electrical ballasts may weigh about 40 pounds. A typical office may have several such fixtures mounted to the ceiling of each room to provide room lighting.
A ceiling mounted fluorescent bulb, such as the bulbs 104, 106 and 108, is only about 50-60% efficient in directing its light downwards to the room below. As illustrated by FIG. 2, if a single ceiling mounted fluorescent bulb 108 in a typically reflective luminaire or reflector 210 is considered to emit light from four quadrants A, B, C, and D, for example, about 30% of the light emitted from quadrant A reaches a room below, about 55% from quadrants B and C is directed downwards and almost 95% from quadrant D is directed downwards so that the end result is approximately 50-60% efficiency. By contrast, a plurality of LEDs 300 mounted in a similar reflective fixture 310 direct most of their light downward to the room below.
With respect to heat dissipation, the fluorescent bulbs 102, 106 and 108 extend the length of box 102 as indicated by the dashed lines for their subassemblies in FIG. 1. With their large surface areas, they very effectively transfer their heat to the surrounding air and subassemblies so that heat dissipation is not a problem for fluorescent lighting fixtures of this kind. By contrast, when a fluorescent bulb is replaced by a series of high power LEDs, such as the LEDs 300 of FIG. 3, as represented by xs in FIG. 1, heat dissipation becomes an issue. In this example, high power means an LED having a current of 125 mA or higher. In most cases, power LEDs for lighting applications will be mounted on metal core printed circuit boards (MCPCB), which will be thermally connected to an isotropic heat sink. Heat flows through the MCPCB to the heat sink by way of conduction. The heat sink diffuses heat to the ambient surroundings by convection. There are three common varieties of heat sinks: flat plates, dip-cast finned heat sinks, and extruded finned heat sinks. A material often used for heat sink construction is aluminum, although copper may be advantageously used for flat-sheet heat sinks.
One approach to heat dissipation is to use a large multivaned or multifinned aluminum heat sink, such as heat sink 320 seen in FIG. 3. Such a heat sink may not be practical in a luminaire fixture retrofit for a number of reasons. A typical 2′ by 4′ fluorescent luminaire light fixture, such as the fixture 100, shown in FIG. 1, may weigh approximately 40 pounds and its top surface 112 mounts flush with the ceiling of the room in which it is to be utilized. By contrast, if one heat sink 320 weighs approximately 8 pounds, then the use of three additional heat sinks 320 would add about 24 pounds to the weight of fixture 100. If the cost of each heat sink 320 is about $40-$50 with shipping from the supplier costing more than $10, then the increased total cost may be prohibitive to many potential purchasers. Additionally, the heat sink would have to be mounted recessed into the ceiling for an LED-based fixture to be mounted flush with the ceiling in a manner compatible with the present mount typical of fluorescent fixtures, such as the 100. Thus, such an approach would not provide a particularly cost effective or physically compatible retrofit with existing fluorescent luminaire light fixtures.
With respect to newly designed LED lighting fixtures having different form factors from standard lighting LED fixtures, there still may be issues with respect to satisfactory dissipation of heat from one or more high power LEDs or even from lower power LEDs where multiple LEDs are employed.